use core::mem::swap;

#[cfg(feature = "serde")]
use serde::{Deserialize, Serialize};

use self::heap_helpers::StaticHeapHole;
pub use self::heap_helpers::StaticHeapPeekMut;
pub use self::heap_iterators::{StaticHeapDrainSorted, StaticHeapIntoIterSorted};
use crate::iterators::{StaticVecDrain, StaticVecIterConst, StaticVecIterMut};
use crate::StaticVec;

mod heap_helpers;
mod heap_iterators;
mod heap_trait_impls;

/// A priority queue implemented as a binary heap, built around an instance of `StaticVec<T, N>`.
///
/// `StaticHeap`, as well as the associated iterator and helper structs for it are direct
/// adaptations of the ones found in the `std::collections::binary_heap` module (including
/// most of the documentation, at least for the functions that exist in both implementations).
///
/// It is a logic error for an item to be modified in such a way that the
/// item's ordering relative to any other item, as determined by the `Ord`
/// trait, changes while it is in the heap. This is normally only possible
/// through `Cell`, `RefCell`, global state, I/O, or unsafe code.
///
/// # Examples
///
/// ```
/// use staticvec::StaticHeap;
///
/// let mut heap = StaticHeap::<i32, 4>::new();
///
/// // We can use peek to look at the next item in the heap. In this case,
/// // there's no items in there yet so we get None.
/// assert_eq!(heap.peek(), None);
///
/// // Let's add some scores...
/// heap.push(1);
/// heap.push(5);
/// heap.push(2);
///
/// // Now peek shows the most important item in the heap.
/// assert_eq!(heap.peek(), Some(&5));
///
/// // We can check the length of a heap.
/// assert_eq!(heap.len(), 3);
///
/// // We can iterate over the items in the heap, although they are returned in
/// // a random order.
/// for x in &heap {
///   println!("{}", x);
/// }
///
/// // If we instead pop these scores, they should come back in order.
/// assert_eq!(heap.pop(), Some(5));
/// assert_eq!(heap.pop(), Some(2));
/// assert_eq!(heap.pop(), Some(1));
/// assert_eq!(heap.pop(), None);
///
/// // We can clear the heap of any remaining items.
/// heap.clear();
///
/// // The heap should now be empty.
/// assert!(heap.is_empty())
/// ```
///
/// ## Min-heap
///
/// Either `core::cmp::Reverse` or a custom `Ord` implementation can be used to
/// make `StaticHeap` a min-heap. This makes `heap.pop()` return the smallest
/// value instead of the greatest one.
///
/// ```
/// use staticvec::StaticHeap;
/// use core::cmp::Reverse;
///
/// // Wrap the values in `Reverse`.
/// let mut heap = StaticHeap::from([Reverse(1), Reverse(5), Reverse(2)]);
///
/// // If we pop these scores now, they should come back in the reverse order.
/// assert_eq!(heap.pop(), Some(Reverse(1)));
/// assert_eq!(heap.pop(), Some(Reverse(2)));
/// assert_eq!(heap.pop(), Some(Reverse(5)));
/// assert_eq!(heap.pop(), None);
/// ```
///
/// # Time complexity
///
/// | [push] | [pop]    | [peek]/[peek\_mut] |
/// |--------|----------|--------------------|
/// | O(1)~  | O(log n) | O(1)               |
///
/// The value for `push` is an expected cost; the method documentation gives a
/// more detailed analysis.
///
/// [push]: #method.push
/// [pop]: #method.pop
/// [peek]: #method.peek
/// [peek\_mut]: #method.peek_mut
#[cfg_attr(feature = "serde", derive(Deserialize, Serialize))]
pub struct StaticHeap<T, const N: usize> {
  pub(crate) data: StaticVec<T, N>,
}

impl<T: Ord, const N: usize> StaticHeap<T, N> {
  /// Creates an empty StaticHeap as a max-heap.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::StaticHeap;
  /// let mut heap = StaticHeap::<i32, 2>::new();
  /// heap.push(4);
  /// ```
  #[inline(always)]
  pub const fn new() -> StaticHeap<T, N> {
    StaticHeap {
      data: StaticVec::new(),
    }
  }

  /// Returns a mutable reference to the greatest item in the StaticHeap, or
  /// `None` if it is empty.
  ///
  /// Note: If the `StaticHeapPeekMut` value is leaked, the heap may be in an
  /// inconsistent state.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::StaticHeap;
  /// let mut heap = StaticHeap::<i32, 4>::new();
  /// assert!(heap.peek_mut().is_none());
  /// heap.push(1);
  /// heap.push(5);
  /// heap.push(2);
  /// {
  ///   let mut val = heap.peek_mut().unwrap();
  ///   *val = 0;
  /// }
  /// assert_eq!(heap.peek(), Some(&2));
  /// ```
  ///
  /// # Time complexity
  ///
  /// Cost is O(1) in the worst case.
  #[inline(always)]
  pub const fn peek_mut(&mut self) -> Option<StaticHeapPeekMut<'_, T, N>> {
    if self.is_empty() {
      None
    } else {
      Some(StaticHeapPeekMut {
        heap: self,
        sift: true,
      })
    }
  }

  /// Pops a value from the end of the StaticHeap and returns it directly without asserting that
  /// the StaticHeap's current length is greater than 0.
  ///
  /// # Safety
  ///
  /// It is up to the caller to ensure that the StaticHeap contains at least one
  /// element prior to using this function. Failure to do so will result in reading
  /// from uninitialized memory.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let mut heap = StaticHeap::from([1, 3]);
  /// unsafe {
  ///   assert_eq!(heap.pop_unchecked(), 3);
  ///   assert_eq!(heap.pop_unchecked(), 1);
  /// }
  /// ```
  ///
  /// # Time complexity
  ///
  /// The worst case cost of `pop_unchecked` on a heap containing *n* elements is O(log n).
  #[inline(always)]
  pub unsafe fn pop_unchecked(&mut self) -> T {
    let mut res = self.data.pop_unchecked();
    if self.is_not_empty() {
      swap(&mut res, self.data.get_unchecked_mut(0));
      self.sift_down_to_bottom(0);
    }
    res
  }

  /// Removes the greatest item from the StaticHeap and returns it, or `None` if it
  /// is empty.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let mut heap = StaticHeap::from([1, 3]);
  /// assert_eq!(heap.pop(), Some(3));
  /// assert_eq!(heap.pop(), Some(1));
  /// assert_eq!(heap.pop(), None);
  /// ```
  ///
  /// # Time complexity
  ///
  /// The worst case cost of `pop` on a heap containing *n* elements is O(log n).
  #[inline(always)]
  pub fn pop(&mut self) -> Option<T> {
    if self.is_empty() {
      None
    } else {
      Some(unsafe { self.pop_unchecked() })
    }
  }

  /// Pushes a value onto the StaticHeap without asserting that
  /// its current length is less than `self.capacity()`.
  ///
  /// # Safety
  ///
  /// It is up to the caller to ensure that the length of the StaticHeap
  /// prior to using this function is less than `self.capacity()`.
  /// Failure to do so will result in writing to an out-of-bounds memory region.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::StaticHeap;
  /// let mut heap = StaticHeap::<i32, 3>::new();
  /// unsafe {
  ///   heap.push_unchecked(3);
  ///   heap.push_unchecked(5);
  ///   heap.push_unchecked(1);
  /// }
  /// assert_eq!(heap.len(), 3);
  /// assert_eq!(heap.peek(), Some(&5));
  /// ```
  ///
  /// # Time complexity
  ///
  /// The expected cost of `push_unchecked`, averaged over every possible ordering of
  /// the elements being pushed, and over a sufficiently large number of
  /// pushes, is O(1). This is the most meaningful cost metric when pushing
  /// elements that are *not* already in any sorted pattern.
  ///
  /// The time complexity degrades if elements are pushed in predominantly
  /// ascending order. In the worst case, elements are pushed in ascending
  /// sorted order and the amortized cost per push is O(log n) against a heap
  /// containing *n* elements.
  ///
  /// The worst case cost of a *single* call to `push_unchecked` is O(n).
  #[inline(always)]
  pub unsafe fn push_unchecked(&mut self, item: T) {
    let old_length = self.len();
    self.data.push_unchecked(item);
    self.sift_up(0, old_length);
  }

  /// Pushes an item onto the StaticHeap, panicking if the underlying StaticVec
  /// instance is already at maximum capacity.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::StaticHeap;
  /// let mut heap = StaticHeap::<i32, 5>::new();
  /// heap.push(3);
  /// heap.push(5);
  /// heap.push(1);
  /// assert_eq!(heap.len(), 3);
  /// assert_eq!(heap.peek(), Some(&5));
  /// ```
  ///
  /// # Time complexity
  ///
  /// The expected cost of `push`, averaged over every possible ordering of
  /// the elements being pushed, and over a sufficiently large number of
  /// pushes, is O(1). This is the most meaningful cost metric when pushing
  /// elements that are *not* already in any sorted pattern.
  ///
  /// The time complexity degrades if elements are pushed in predominantly
  /// ascending order. In the worst case, elements are pushed in ascending
  /// sorted order and the amortized cost per push is O(log n) against a heap
  /// containing *n* elements.
  ///
  /// The worst case cost of a *single* call to `push` is O(n).
  #[inline(always)]
  pub fn push(&mut self, item: T) {
    // Deferring to our own `push_unchecked` which defers to `StaticVec::push_unchecked`
    // is slower here than just calling `StaticVec::push` which calls `StaticVec::push_unchecked`
    // anyways.
    let old_length = self.len();
    self.data.push(item);
    self.sift_up(0, old_length);
  }

  /// Consumes the StaticHeap and returns a StaticVec in sorted (ascending) order.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let mut heap = StaticHeap::<i32, 8>::from([1, 2, 4, 5, 7]);
  /// heap.push(6);
  /// heap.push(3);
  /// let vec = heap.into_sorted_staticvec();
  /// assert_eq!(vec, [1, 2, 3, 4, 5, 6, 7]);
  /// ```
  #[inline]
  pub fn into_sorted_staticvec(mut self) -> StaticVec<T, N> {
    let mut end = self.len();
    while end > 1 {
      end -= 1;
      self.data.swap(0, end);
      self.sift_down_range(0, end);
    }
    self.into_staticvec()
  }

  // The implementations of sift_up and sift_down use unsafe blocks in
  // order to move an element out of the vector (leaving behind a
  // hole), shift along the others and move the removed element back into the
  // vector at the final location of the hole.
  // The `StaticHeapHole` type is used to represent this, and make sure
  // the hole is filled back at the end of its scope, even on panic.
  // Using a hole reduces the constant factor compared to using swaps,
  // which involves twice as many moves.
  #[inline]
  fn sift_up(&mut self, start: usize, position: usize) {
    unsafe {
      // Take out the value at `position` and create a hole.
      let mut hole = StaticHeapHole::new(&mut self.data, position);
      while hole.pos() > start {
        let parent = (hole.pos() - 1) / 2;
        if hole.elt() <= hole.get(parent) {
          break;
        }
        hole.move_to(parent);
      }
    }
  }

  /// Takes an element from `position` and moves it down the heap,
  /// while its children are larger.
  #[inline]
  fn sift_down_range(&mut self, position: usize, end: usize) {
    unsafe {
      let mut hole = StaticHeapHole::new(&mut self.data, position);
      let mut child = 2 * position + 1;
      while child < end {
        let right = child + 1;
        // compare with the greater of the two children
        if right < end && hole.get(child) <= hole.get(right) {
          child = right;
        }
        // if we are already in order, stop.
        if hole.elt() >= hole.get(child) {
          break;
        }
        hole.move_to(child);
        child = 2 * hole.pos() + 1;
      }
    }
  }

  /// Takes an element from `position` and moves it all the way down the heap,
  /// then sifts it up to its position.
  ///
  /// Note: This is faster when the element is known to be large / should
  /// be closer to the bottom.
  #[inline]
  fn sift_down_to_bottom(&mut self, mut position: usize) {
    let end = self.len();
    let start = position;
    unsafe {
      let mut hole = StaticHeapHole::new(&mut self.data, position);
      let mut child = 2 * position + 1;
      while child < end {
        let right = child + 1;
        // compare with the greater of the two children
        if right < end && hole.get(child) <= hole.get(right) {
          child = right;
        }
        hole.move_to(child);
        child = 2 * hole.pos() + 1;
      }
      position = hole.position;
    }
    self.sift_up(start, position);
  }

  #[inline(always)]
  fn rebuild(&mut self) {
    let mut n = self.len() / 2;
    while n > 0 {
      n -= 1;
      self.sift_down_range(n, self.len());
    }
  }

  /// Appends `self.remaining_capacity()` (or as many as available) items from `other` to `self`.
  /// The appended items (if any) will no longer exist in `other` afterwards (which is to say,
  /// `other` will be left empty.)
  ///
  /// The `N2` parameter does not need to be provided explicitly, and can be inferred directly from
  /// the constant `N2` constraint of `other` (which may or may not be the same as the `N`
  /// constraint of `self`.)
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// // We give the two heaps arbitrary capacities for the sake of the example.
  /// let mut a = StaticHeap::<i32, 9>::from([-10, 1, 2, 3, 3]);
  /// let mut b = StaticHeap::<i32, 18>::from([-20, 5, 43]);
  /// a.append(&mut b);
  /// assert_eq!(a.into_sorted_staticvec(), [-20, -10, 1, 2, 3, 3, 5, 43]);
  /// assert!(b.is_empty());
  /// ```
  #[inline(always)]
  pub fn append<const N2: usize>(&mut self, other: &mut StaticHeap<T, N2>) {
    if other.is_empty() {
      return;
    }
    self.data.append(&mut other.data);
    self.rebuild();
  }

  /// Returns an iterator which retrieves elements in heap order.
  /// The retrieved elements are removed from the original heap.
  /// The remaining elements will be removed on drop in heap order.
  ///
  /// Note:
  /// * `drain_sorted()` is O(n log n); much slower than `drain()`. You should use the latter for
  ///   most cases.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let mut heap = StaticHeap::from([1, 2, 3, 4, 5]);
  /// assert_eq!(heap.len(), 5);
  /// drop(heap.drain_sorted()); // removes all elements in heap order
  /// assert_eq!(heap.len(), 0);
  /// ```
  #[inline(always)]
  pub const fn drain_sorted(&mut self) -> StaticHeapDrainSorted<'_, T, N> {
    StaticHeapDrainSorted { inner: self }
  }
}

impl<T, const N: usize> StaticHeap<T, N> {
  /// Returns an iterator visiting all values in the StaticHeap's underlying StaticVec, in
  /// arbitrary order.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let heap = StaticHeap::from(staticvec![1, 2, 3, 4]);
  /// // Print 1, 2, 3, 4 in arbitrary order
  /// for x in heap.iter() {
  ///   println!("{}", x);
  /// }
  /// ```
  #[inline(always)]
  pub const fn iter(&self) -> StaticVecIterConst<'_, T, N> {
    self.data.iter()
  }

  /// Returns a mutable iterator visiting all values in the StaticHeap's underlying StaticVec, in
  /// arbitrary order.
  ///
  /// **Note:** Mutating the elements in a StaticHeap may cause it to become unbalanced.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let mut heap = StaticHeap::from([1, 2, 3, 4]);
  /// for i in heap.iter_mut() {
  ///   *i *= 2;
  /// }
  /// // Prints "[2, 4, 6, 8]", but in arbitrary order
  /// println!("{:?}", heap);
  /// ```
  #[inline(always)]
  pub const fn iter_mut(&mut self) -> StaticVecIterMut<'_, T, N> {
    self.data.iter_mut()
  }

  /// Returns an iterator which retrieves elements in heap order.
  /// This method consumes the original StaticHeap.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let heap = StaticHeap::from([1, 2, 3, 4, 5]);
  /// assert_eq!(
  ///   heap.into_iter_sorted().take(2).collect::<StaticVec<_, 3>>(), staticvec![5, 4]
  /// );
  /// ```
  #[inline(always)]
  pub const fn into_iter_sorted(self) -> StaticHeapIntoIterSorted<T, N> {
    StaticHeapIntoIterSorted { inner: self }
  }

  /// Returns the greatest item in the StaticHeap, or `None` if it is empty.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let mut heap = StaticHeap::<i32, 7>::new();
  /// assert_eq!(heap.peek(), None);
  /// heap.push(1);
  /// heap.push(5);
  /// heap.push(2);
  /// assert_eq!(heap.peek(), Some(&5));
  /// ```
  ///
  /// # Time complexity
  ///
  /// Cost is O(1) in the worst case.
  #[inline(always)]
  pub fn peek(&self) -> Option<&T> {
    self.data.get(0)
  }

  /// Returns the maximum number of elements the StaticHeap can hold.
  /// This is always equivalent to its constant generic `N` parameter.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let mut heap = StaticHeap::<i32, 100>::new();
  /// assert!(heap.capacity() >= 100);
  /// heap.push(4);
  /// ```
  #[inline(always)]
  pub const fn capacity(&self) -> usize {
    self.data.capacity()
  }

  /// Returns the remaining capacity (which is to say, `self.capacity() - self.len()`) of the
  /// StaticHeap.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let mut heap = StaticHeap::<i32, 100>::new();
  /// heap.push(1);
  /// assert_eq!(heap.remaining_capacity(), 99);
  /// ```
  #[inline(always)]
  pub const fn remaining_capacity(&self) -> usize {
    self.data.remaining_capacity()
  }

  /// Returns the total size of the inhabited part of the StaticHeap (which may be zero if it has a
  /// length of zero or contains ZSTs) in bytes. Specifically, the return value of this function
  /// amounts to a calculation of `size_of::<T>() * self.length`.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let x = StaticHeap::<u8, 8>::from([1, 2, 3, 4, 5, 6, 7, 8]);
  /// assert_eq!(x.size_in_bytes(), 8);
  /// let y = StaticHeap::<u16, 8>::from([1, 2, 3, 4, 5, 6, 7, 8]);
  /// assert_eq!(y.size_in_bytes(), 16);
  /// let z = StaticHeap::<u32, 8>::from([1, 2, 3, 4, 5, 6, 7, 8]);
  /// assert_eq!(z.size_in_bytes(), 32);
  /// let w = StaticHeap::<u64, 8>::from([1, 2, 3, 4, 5, 6, 7, 8]);
  /// assert_eq!(w.size_in_bytes(), 64);
  /// ```
  #[inline(always)]
  pub const fn size_in_bytes(&self) -> usize {
    self.data.size_in_bytes()
  }

  /// Consumes the StaticHeap and returns the underlying StaticVec
  /// in arbitrary order.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let heap = StaticHeap::from(staticvec![1, 2, 3, 4, 5, 6, 7]);
  /// let vec = heap.into_staticvec();
  /// // Will print in some order
  /// for x in &vec {
  ///   println!("{}", x);
  /// }
  /// ```
  #[inline(always)]
  pub fn into_staticvec(self) -> StaticVec<T, N> {
    self.data
  }

  /// Returns the length of the StaticHeap.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let heap = StaticHeap::from(staticvec![1, 3]);
  /// assert_eq!(heap.len(), 2);
  /// ```
  #[inline(always)]
  pub const fn len(&self) -> usize {
    self.data.len()
  }

  /// Returns true if the current length of the StaticHeap is 0.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let mut heap = StaticHeap::<i32, 28>::new();
  /// assert!(heap.is_empty());
  /// ```
  #[inline(always)]
  pub const fn is_empty(&self) -> bool {
    self.len() == 0
  }

  /// Returns true if the current length of the StaticHeap is greater than 0.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let mut heap = StaticHeap::<i32, 2>::new();
  /// heap.push(1);
  /// assert!(heap.is_not_empty());
  /// ```
  // Clippy wants `!is_empty()` for this, but I prefer it as-is. My question is though, does it
  // actually know that we have an applicable `is_empty()` function, or is it just guessing? I'm not
  // sure.
  #[allow(clippy::len_zero)]
  #[inline(always)]
  pub const fn is_not_empty(&self) -> bool {
    self.len() > 0
  }

  /// Returns true if the current length of the StaticHeap is equal to its capacity.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let mut heap = StaticHeap::<i32, 4>::new();
  /// heap.push(3);
  /// heap.push(5);
  /// heap.push(1);
  /// heap.push(2);
  /// assert!(heap.is_full());
  /// ```
  #[inline(always)]
  pub const fn is_full(&self) -> bool {
    self.len() == N
  }

  /// Returns true if the current length of the StaticHeap is less than its capacity.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let mut heap = StaticHeap::<i32, 4>::new();
  /// heap.push(3);
  /// heap.push(5);
  /// heap.push(1);
  /// assert!(heap.is_not_full());
  /// ```
  #[inline(always)]
  pub const fn is_not_full(&self) -> bool {
    self.len() < N
  }

  /// Clears the StaticHeap, returning an iterator over the removed elements.
  ///
  /// The elements are removed in arbitrary order.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let mut heap = StaticHeap::from(staticvec![1, 3]);
  /// assert!(heap.is_not_empty());
  /// for x in heap.drain() {
  ///   println!("{}", x);
  /// }
  /// assert!(heap.is_empty());
  /// ```
  #[inline(always)]
  pub fn drain(&mut self) -> StaticVecDrain<'_, T, N> {
    self.data.drain_iter(..)
  }

  /// Drops all items from the StaticHeap.
  ///
  /// # Examples
  ///
  /// Basic usage:
  /// ```
  /// # use staticvec::*;
  /// let mut heap = StaticHeap::from(staticvec![1, 3]);
  /// assert!(heap.is_not_empty());
  /// heap.clear();
  /// assert!(heap.is_empty());
  /// ```
  #[inline(always)]
  pub fn clear(&mut self) {
    self.data.clear();
  }
}